1. Field of the Invention
The present invention relates to a method of fabricating a thermoelectric sensor. More particularly, the present invention relates to increasing the length of the thermocouple element line and decreasing the heat conductivity by using a zigzag structure, or a meander structure, of the thermocouple element line, as well as etching the silicon substrate by using front side Si bulk set etching. This invention will not only increase the gross die throughput of the silicon wafer, but also make a resister to be treated as a heater on the sensor membrane structure for electrical calibration of the device.
2. Description of the Prior Art
Infrared radiation detectors measure an object temperature by receiving and counting the infrared ray intensity generated from the object. It is a so-called non-contact temperature measurement. The typical applications include in-situ monitoring of semiconductor process steps, infrared spectroscopy, detection of various gases and liquids in industrial process environments, and measurement of temperature distribution in house for air conditioner. These applications also include medical thermography and clinical tympanic thermometers, and security applications for a passive intrusion detector. The operation principle basically is that the infrared rays radiate onto the detector, causing the temperature of a radiation absorber area to rise. Then this temperature increase will cause a change in physical nature of the device. This change can be converted into a change of electrical output. The common sensors are known as pyroelectric sensor, bolometer, and thermopile.
The pyroelectric sensor is made of ceramics of polymer, and it needs to be carefully treated during assembly and packaging. The manufacturing cost of pyroelectric sensors is comparatively high than thermoelectric sensors, since a thermoelectric sensor can be batch fabricated by using semiconductor mass production line. On the other hand, the bolometer sensor needs bias to measure the variation of its resistance and 1/f noise will occur due to such bias reading. The thermopile, i.e., the thermoelectric sensor, is a group of thermocouples connected in series. A thermocouple consists of a pair of dissimilar conductors so joined at two points that an electromotive force is developed by the thermoelectric effect when these two points, i.e., junctions, are at different temperatures. The thermoelectric sensor requires no additional bias, and is useful over a wide range of ambient temperatures. Because the thermoelectric sensor can be fabricated by semiconductor process technology, it is easily interlaced with monolithic integrated circuits. As a result, the thermoelectric sensor shows strong market potential and cost competitiveness.
From the view point of sensor function, if the device can include a thermoelectric sensing element and amplifying circuit, then the function and performance of the device will be increased, and the noise of signal interface will be decreased as well. It points out that the advantage of developing a compatible thermoelectric sensing element with the standard CMOS process. This arrangement provides the possibility of integrating a thermoelectric sensing element and signal processing circuit into a monolithic integrated circuit (IC) sensor.
According to the post-process that thermoelectric sensors in the prior art were usually possessed of the freestanding membrane structure made by back side silicon wet etching, but this way exhibits shortcomings as below:
Referring to FIG. 1A, a cross-sectional view of the rim of a thermoelectric sensor is shown. A silicon substrate 1, a close membrane 2, a plurality of first thermocouple element lines 3, isolation layer 4, a plurality of second thermocouple element lines 5, and radiation absorber layer 6 are provided. The radiation absorber layer 6 that is isolated to the isolation layer 4' couples with the first thermoelectric element line 3 and second thermocouple element line 5. The close membrane 2, first thermocouple element line 3, isolation layer 4, and second thermocouple element line 5 are symmetrical as a result of the rim of thermoelectric sensor.
FIB. 1B shows the connection of interconnects according to the thermoelectric sensor of FIG. 1A. The opening of first thermocouple element line 3 couples the extremity of second thermocouple element line 5 with heated junction H nearby radiation absorber layer 6 by way of series connection, and cold junction C is distant from radiation absorber layer 6. The first cold junction C of first thermocouple element line 3 couples electrically to the first metal pad 7, and the last cold junction C of second thermocouple element line 5 couples electrically to the second metal pad 8.
The heated junction H is located above close membrane 2 and located beneath radiation absorber layer 6, and its temperature will be increased due to the heat coming from radiation absorber layer 6. This radiation absorber layer 6 will be heated when it received infrared radiation. The cold junction C is located on silicon substrate 1. Its temperature will be the same as the ambient temperature, because silicon substrate has high solid conductance and heat from cold junction dissipates through silicon substrate 1. In order to form the close membrane 2, the silicon anisotropic wet etching process is applied to remove the underneath silicon substrate from close membrane 2. If the etching window is formed on the backside of silicon substrate, then the etching solution will attack silicon from the backside. A larger etching window area is necessary to define the area of close membrane 2, due to the different etching rate of silicon crystal facet. It means the overall device size will become much larger than the defined close membrane 2 area, since the backside etching window is larger than the defined close membrane 2 area, and the device must be larger than the backside etching window. However, if the etching window is formed on the front side of silicon substrate, then the induced cavity will be like the one shown in FIG. 2A. It means the overall device size will become almost the same as the defined close membrane 2 area. In such case, a cantilever beam of a 4-arm bridge will be generally used to support the structure of radiation absorber layer and the group of thermocouples. Ascribing to the different sizes of device made by front side or backside silicon anisotropic wet etching, the gross die of thermoelectric sensor from the wafer made by front side wet etching will be larger than the one made by backside wet etching.
In general, the characteristics of thermoelectric sensor express with responsivity in volts per watt (Rv), Johnson noise (V.sub.j), Noise Equivalent Power (NEP), and specific detectivity (D*). The corresponding formula is according to the following equation: ##EQU1##
Where N is the number of thermocouples, and a is the seebeck coefficient (V/.degree. C.). The G.sub.s, G.sub.g, G.sub.r are separately the thermal conductivity of solid, gas, and radiation, respectively. The k is Boltzmann's constant, T is the absolute temperature (.degree. K.), R is the electrical resistance, .DELTA.f is the bandwidth of the amplifier, and A is the area of radiation absorber layer.
According to the aforementioned illustration, thermoelectric sensor measures the temperature of an object, the sensor performance depends on the quantity of output signal and the sensitivity of temperature variation of a measured object. The responsivity Rv is the output voltage for unit input radiation power. It represents the output efficiency of a sensor. Noise equivalent power NEP is the input power when the output voltage of thermopile is equal to the noise level. The specific detectivity D* is the inverse number of NEP and normalized by the radiation absorber area and signal frequency. D* is the index of the sensor that express how small a signal can be detected. Because the thermal conductivity of structure materials of thermoelectric sensor is inversely proportional to values of Rv and D*, improvements to Rv and D* can be done by employing the lower thermal conductivity of the thermocouple element line or increasing the length of the thermocouple element line.
Referring to FIG. 1B, the first thermocouple element line 3 and the second thermocouple element line 5 are straight lines. The thermocouple element line 5 is usually made of aluminum that has high thermal conductivity. However, for the thermoelectric sensor fabricated by standard semiconductor CMOS process, the first thermocouple element line 3 is polysilicon and the second thermocouple element line 5 is aluminum. Therefore, the sensor performance is limited by the factor of high thermal conductivity of the second thermocouple element line 5. The overall thermal conductance decreases by increasing length of thermocouple element line, but the overall resistance of thermopile increases too. Because the resistance of polysilicon is high, the resistance of thermopile increases too. Because the resistance of polysilicon is high, the resistance of first thermocouple element line will increase rapidly as its length increases. As a result, the overall sensor performance is not simply increased in proportion to increment of length of thermocouple element line. The present invention discloses a zigzag structure of the second thermocouple element line to decrease the thermal conductance of the second thermocouple element line, while the length of the first thermocouple element line is kept the same. In such invention, the sensor performance can be promoted without changing the size of sensor or the size of membrane.
The general thermoelectric sensor measures the ambient temperature by using an off-chip temperature sensor, such as thermister, transistor or diode. The present invention applies the same material with the first thermocouple element line or the second thermocouple element line to form an on-chip temperature sensitive resistor that can be used to measure the ambient temperature. Besides, to avoid rising temperature of silicon substrate due to the dissipated heat power from the bias reading of this on-chip temperature sensitive resistor, the resistance of this on-chip resistor is designed to be greater than 100 K.OMEGA..
Furthermore, in the non-contact temperature measurement, the object temperature can be calculated from the output voltage value of thermoelectric sensor. The output voltage, or responsivity, may decrease or deteriorate due to the aging of thermocouple materials caused by strong environment variation and storage damage. This effect will lead to the measurement deviation in the long term. It also costs the manufacturer to use expensive electrical circuits to calibrate these effects for commercial product. The present invention provides a method that a heater resistor is made on the neighboring area of a hot junction and is used to calibrate the drift effect of thermocouple materials by following electrical calibration method.
The prior thermoelectric sensor comprises metal can of four pins that has two pins for thermoelectric sensor and two pins for off-chip ambient temperature sensor. The present invention provides a metal can package of the thermoelectric sensor in which it has 5 pins or 6 pins. It provides two pins for thermoelectric sensor, two pins for on-chip temperature sensitive resistor, and two pins for the heater resistor, while the temperature sensitive resistor and heater resistor can share a common ground. The two pins of thermoelectric sensor are isolated from the metal can to reduce the noise influence.
For prior thermoelectric sensors, some significant patents are as follows:
(1) U.S. Pat. No. 4,665,276, Thomas Elbel, Jurgen Muller, Friedemann Volklein, "Thermoelectric Sensor" PA1 (2) U.S. Pat. No. 5,100,479, Kensall D. Wise, Khalil Najati, "Thermopile, Infrared Detector with Semiconductor Supporting RIM" PA1 (3) U.S. Pat. No. 4,456,919, Katsuhiko Tomita, Tatsuo Shimizu, Masaichi Bandoh, "Thermopile Type Detector with Temperature Sensor for Cold Junction" PA1 (1) G. R. Lahij i and K. D. Wise, "A batch-fabricated silicon thermopile infrared detector", IEEE Trans. Electron Devices ED-29, pp 14-22, (1982). PA1 (2) R. Lenggenhager, H. Baltes, J. Peer and M. Forster, "Thermoelectric infrared sensors by CMOS technology", IEEE Electron Device Letters 13, 454, (1992). PA1 (3) T. Akin, Z. Olgun, O. Akar, and H. Kulah "An integrated thermopile structure with high responsivity using any stndard CMOS process", Sensors and Actuators A66, pp.218-224 (1998). PA1 (4) H. Baltes, "CMOS as Sensor Technology", Sensors and Actuators A37-38, pp.51-56 (1993). PA1 (5) HL-PLANAR Technik Catalog of Thermosensors (TS 10060), HL-Planartechnik GmbH, Hauert 13 44 227 Dortmund, Germany. PA1 (1) Selecting a material with lower thermal conductivity coefficient as thermocouple element line and making use of photolithographic method to form a meander or zigzag structure of thermocouple element line. It could increase the length of thermocouple element line and decrease the heat conductance, and then increase the sensitivity and performance of thermoelectric sensor without increasing the die size. PA1 (2) Selecting titanium alloy with a lower thermal conductivity as a material thermocouple element line and decreasing the solid thermal conductance thereafter. PA1 (3) Fabricating a resistor to be treated as a heater on the neighboring area of hot junction is used for electrical calibration against the aging of thermoelectric and other structure materials and environmental effects, and then increasing the precision of non-contact temperature measurement in a long term of operation. PA1 (4) Employing titanium thin film and its relative alloy from the semiconductor industrial process technology to the radiation absorber layer. PA1 (5) Opening an etching hole on the free-standing, or floating, membrane of radiation absorber layer and isolation layer to shorten the etching time and to increase yield. PA1 (6) Employing front side silicon anisotropic wet etching technique to remove the underneath silicon substrate of open membrane structure of thermoelectric sensor. Then heated junction is suspended or thermal isolated from silicon substrate, so as to increase signal output. The method provides higher gross die, easier package, easier process than prior art, and comparatively reduces etching time and possible damage of sensing element. PA1 (7) Selecting the same material as first or second thermocouple element line as an on-chip temperature sensitive resistor, and resistance of this resistor is greater than 100 K.OMEGA.. PA1 (8) Forming a reflective mirror to cover the front side etching window to avoid influence of reflected radiation from Si (111) facet of underneath v groove. This reflective mirror plate is made at the same time of making the thermoelectric sensor structure. This reflective mirror plate comprises aluminum film for reflection. PA1 (9) Selecting 5 pins or 6 pins of metal can.
The U.S. Pat. No. 4,665,276 discloses a thermoelectric sensor of the backside wet etched membrane structure comprising a thin-film thermopile, and providing Be, Sb as the thermoelectric materials.
The U.S. Pat. No. 5,100,479 discloses a thermoelectric sensor consisting of a heavy doped silicon-supporting rim. The rim supports a series of polysilicon and metal thermocouples. It also serves as a good thermal conductor for supporting the cold junctions and removing the heat from the cold junctions.
The means of thermoelectric sensor according to the aforementioned two patents employ close membrane structure of backside wet etching, but the present invention employs open membrane structure of front side wet etching.
The U.S. Pat. No. 4,456,919 discloses a thermoelectric sensor with on-chip diode or transistor that is made at the same fabrication procedure of thermoelectric sensor. This diode or transistor is used to detect ambient temperature for temperature compensation purposes. The present invention uses the first thermocouple element line or second thermocouple element line to form an on-chip temperature sensitive resistor, i.e., a thermister. This on-chip temperature sensitive resistor can be made at the same fabrication procedure of thermoelectric sensor.
Reference in this regard is made to the following articles:
According to aforementioned papers, Lahiji and Wise propose a close membrane structure comprising thermoelectric materials of Au and polysilicon, and use the backside silicon anisotropic wet etching to form the close membrane structure. Lenggenhager et al. provide the thermoelectric material of aluminum and polysilicon, and employ open membrane structure by front side silicon anisotropic wet etching. The radiation absorber layer is composed of Sio.sub.2 and SiN. Kulah et. al, provide a structure that the open membrane structure made by front side silicon anisotropic wet etching, while the thermoelectric materials use n-poly and p+-active layers of standard CMOS process. However the electrochemical etch-stop technique is necessary to protect p+-active layer from wet etching during removing underneath silicon substrate process. This approach leads to a complex process and causes a decrease in the yield of gross die.
Besides, the method of the reference paper (4) is disclosed that a heater resistor is placed on the membrane and used to measure the thermal conductance of the structure. But, in the present invention, a heater resistor is made on the neighboring area of hot junction is used for electrical calibration against the aging of thermoelectric materials. The reference materials (5) employ an on-chip Ni thermister as the temperature sensor, while the present invention employs the material as same as first or second thermocouple element line to form the on-chip temperature sensitive resistor. This on-chip temperature sensitive resistor and the thermopile structure are fabricated at the same time.
When the normalized output is derived versus the different incident radiation angle, we find the 100% normalized output happened at the radiation incident angle of about .+-.10.degree. deviated from the normal direction of the plane of radiation absorber layer, and 85% normalized output is observed in the case of incident angle of about 0.degree.. This fact is ascribed to some reflected radiation energy from the Si (111) facet of v-groove cavity being absorbed by the radiation absorber layer. The present invention discloses a reflective mirror to cover the front side etching window to avoid the radiation penetrate through the front side etching window and incident on Si (111) facet, then reflected and incident onto radiation absorber layer. This reflective mirror plate is located at the position of the front side etching window, and can be made at the same time of making the thermoelectric sensor structure. This reflective mirror plate comprises aluminum for the reflection purpose.
The present invention is a cost-effective method. The advantages of polysilicon/metal thermocouples are simple to be fabricated by standard CMOS process. How to promote the gross die, yield, and upgrade the sensor performance is the subject matter of the present invention.